Background and Aims: The duration of the protection of hepatitis B vaccination for infants and the necessity of a booster dose administration is unknown. The aim of the present study was to evaluate the persistence of seroprotection after hepatitis B virus (HBV) vaccination in order to determine the necessity of a single booster dose in 5- to 7-year-old children. Methods: This clinical trial study was conducted from 2004 to 2005.The study population included all children aged 5 to 7 years old in the Kohgiloyeh and Boyerahmad province who had been vaccinated starting at birth with hepatitis B vaccine. Among these children, 729 were selected via a multiple-stage sampling method. Parents gave their informed consent, and blood specimens (3 ml) were obtained from children. Hepatitis B surface antibody (HBsAb) and hepatitis B surface antigen (HBsAg) were determined by enzyme-linked immunosorbent assay (ELISA). Subjects with nonprotective titer levels (< 10 mIU/ml) received a booster does of the DNA recombinant vaccine. Four weeks after the administration of a booster dose, the antibody to HBsAg (anti-HBs) titer was measured. Data were analyzed using SPSS software, and analyses included chi-square, ANOVA, and independent-samples and paired-samples t-tests. Results: 615 children (84.4%) had a protective antibody titer. The mean antibody titer was 230.5 ± 308.9 IU/ml, with a range of 10.6 to 1175 IU/ml. 15.6% of subjects had a nonprotective antibody titer, and the mean antibody titer was 4.97 ± 3.5 IU/ml. All subjects were HBsAg negative. No statistically significant differences were found by sex or by urban versus rural area of residence. The seroprotection rates significantly decreased by as the age of the children increased.Following the booster dose, 78.1 % of the children had a protective titer, and the mean titer significantly increased from 4.97 ± 3.5 at birth to 332.1 ± 402 IU/ml after the booster (P < 0.001). Conclusions: According to our results, the proper response of the immune system to a booster dose of HBV at 5 to 7 years of age reveals that immunologic memory is good after primary vaccination. Administration of booster dose does not appear to be necessary at these ages even though these children have a greater possibility of exposure to HBV in school age.